Gas turbines are known to comprise the following elements: a compressor for compressing air; a combustor for producing a hot gas by burning fuel in the presence of the compressed air produced by the compressor; and a turbine for expanding the hot gas produced by the combustor. Gas turbines are known to emit undesirable oxides of nitrogen (NO.sub.x) and carbon monoxide (CO). One factor known to affect NO.sub.x emission is combustion temperature. The amount of NO.sub.x emitted is reduced as the combustion temperature is lowered. However, higher combustion temperatures are desirable to obtain higher efficiency and CO oxidation.
Two-stage combustion systems have been developed that provide efficient combustion and reduced NO.sub.x emissions. In a two-stage combustion system, diffusion combustion is performed at the first stage for obtaining ignition and flame stability. Premixed combustion is performed at the second stage to reduce NO.sub.x emissions.
The first stage, referred to hereinafter as the "pilot" stage, is normally a diffusion-type burner and is, therefore, a significant contributor of NO.sub.x emissions even though the percentage of fuel supplied to the pilot is comparatively quite small (often less than 10% of the total fuel supplied to the combustor). The pilot flame has thus been known to limit the amount of NO.sub.x reduction that could be achieved with this type of combustor. In a diffusion combustor, the fuel and air are mixed in the same chamber in which combustion occurs (i.e., a combustion chamber).
Pending U.S. patent application Ser. No. 08/759,395, assigned to the same assignee hereunder (the '395 application) and incorporated herein by reference, discloses a typical prior art gas turbine combustor. As shown in FIG. 1 herein, combustor 100 comprises a nozzle housing 6 having a nozzle housing base 5. A diffusion fuel pilot nozzle 1, having a pilot fuel injection port 4, extends through nozzle housing 6 and is attached to nozzle housing base 5. Main fuel nozzles 2, each having at least one main fuel injection port 3, extend substantially parallel to pilot nozzle 1 through nozzle housing 6 and are attached to nozzle housing base 5. Fuel inlets 16 provide fuel 102 to main fuel nozzles 2. A main combustion zone 9 is formed within a liner 19. A pilot cone 20, having a diverged end 22, projects from the vicinity of pilot fuel injection port 4 of pilot nozzle 1. A pilot flame zone 23 is formed within pilot cone 20 adjacent to main combustion zone 9.
Compressed air 101 from compressor 50 flows between support ribs 7 through main fuel swirlers 8. Each main fuel swirler 8 is substantially parallel to pilot nozzle 1 and adjacent to main combustion zone 9. Within each main fuel swirler 8, a plurality of swirler vanes 80 generate air turbulence upstream of main fuel injection ports 3 to mix compressed air 101 with fuel 102 to form a fuel/air mixture 103. Fuel/air mixture 103 is carried into main combustion zone 9 where it combusts. Compressed air 101 also enters pilot flame zone 23 through a set of stationary turning vanes 10 located inside pilot swirler 11. Compressed air 12 mixes with pilot fuel 30 within pilot cone 20 and combusts in pilot flame zone 23.
FIG. 2 shows a cross-sectional view of combustor 100 taken along line 2--2 of FIG. 1. As shown in FIG. 2, pilot nozzle 1 is surrounded by a plurality of main fuel nozzles 2. Pilot swirler 11 surrounds pilot nozzle 1. A main fuel swirler 8 surrounds each main fuel nozzle 2. Pilot swirler 11 forms an annulus 18 with liner 19. Fuel/air mixture 103 flows through main fuel swirlers 8 (out of the page) into main combustion zone 9 (not shown in FIG. 2). Compressed air 101 flows through annulus 18 (out of the page) in the space between main fuel swirlers 8.
Note that compressed air 101 in annulus 18 is not mixed with any fuel and does not flow into main combustion zone 9. Thus, an appreciable volume of compressed air within the main stage is wasted (i.e., not mixed with fuel before main stage combustion. Since, in a premix combustor, the fuel and air are mixed before combustion occurs, the greater the mass of the compressed air 101 that is mixed with fuel 102 in the main stage, the leaner the fuel/air mixture 103 (for a constant mass of fuel 102) that will flow into main combustion zone 9. It is known that leaner, more homogeneous fuel/air mixtures burn cooler and more evenly, thus decreasing NO.sub.x and CO emissions.
While gas turbine combustors such as the combustor disclosed in the '395 application have been developed to reduce NO.sub.x and CO emissions, current environmental concerns demand even greater reductions. Thus, there is a need in the art for a gas turbine combustor having a premix section that reduces NO.sub.x and CO emissions by providing leaner, more homogeneous fuel/air mixtures for main stage combustion.